MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.6. BAKERY PRODUCTS MICROBIOLOGY

15.6.4. Microbial Spoilage of Bakery Products

Rope (ropy) spoilage of bread. The so-called "rope" or ropy spoilage of bread is of a microbiological nature. This was first discovered in 1885 by G. Laurent. The affected bread features a moist, smeary crumb of yellow or brown color, which, when broken or sliced, forms thin, long, stringy, silver threads. Stale bread turns entirely into a brown, smeary mass. As a rule, wheat bread is most susceptible to spoilage, as its crumb has a low acidity. There are four stages in The Development of ropy bread spoilage: barely perceptible (a very faint odor of fruit rot); mild (the odor of rot is clearly perceptible); moderate (stickiness and darkening of the crumb are observed); severe (the odor becomes repulsive, the crumb is dark and pulls into thin silver threads). Favorable conditions for the development of ropy bread include high air humidity, bread storage temperatures above 20 °C, and a dough pH of about 6.5. In an acidic environment at pH 4.5–4.9, these Bacteria do not multiply.

The causative agents of ropy bread are aerobic spore-forming bacilli of the species Bacillus subtilis, whose spores withstand the baking Temperature of bread. The disease can also be caused by spore-forming bacteria of the species B. mycoides, B. megatherium, and others.

Bacillus subtilis consists of rods with rounded ends, measuring (2–3) × (0.6–0.8) µm, Gram-positive, and motile in young cultures. They form centrally located elliptical endospores. These bacteria possess high proteolytic and amylolytic activity—they liquefy gelatin and hydrolyze casein and starch. They reduce nitrates to nitrites and are catalase-positive. Bacillus subtilis spores are quite heat-resistant: they are destroyed at 100 °C in 6 hours, at 113 °C in 45 minutes, at 125 °C in 10 minutes, and at 130 °C instantly. Therefore, the Spore Forms of this bacterial species easily survive the baking heat of the dough piece.

The issue regarding The impact of spore-forming bacilli on human health is of great importance. Ropy spoilage alters the physicochemical, biological, and organoleptic properties of bread, causing it to lose its nutritional value and become unfit for consumption. Furthermore, bread affected by ropy spoilage is toxic and causes the death of 70% of experimental animals (mice).

Measures for Preventing Ropy Bread

One of the essential preventive measures against ropy bread is early Diagnosis, which allows for the timely detection of the pathogen. Based on the contamination of flour with spores of potato and hay bacilli, it is divided into three categories: uninfected, mildly infected, and heavily infected. For top-grade and first-grade flour, a "heavily infected" rating requires reclassifying it as rejected, while mildly infected flour of these grades is recommended for use only in confectionery and small baked goods. Second-grade and whole-grain wheat flour rated as "heavily infected" should be used exclusively for rye-wheat bread varieties.

Control measures against ropy bread boil down, on the one hand, to creating conditions that inhibit the Development of the pathogens, and on the other hand, to eliminating sources of contamination through disinfection.

Methods for suppressing the proliferation of B. subtilis in bread are based on their biological characteristics, specifically their sensitivity to changes in environmental acidity. In an acidic environment, the multiplication of these bacteria slows down. Consequently, bread plants and bakeries employ chemical and biological methods to increase medium acidity.

Chemical agents include lactic, acetic, and propionic acids and their salts. Instead of food-grade lactic acid, whey with an acidity of 100–130 °T can be used in an amount equal to 20% of the flour mass.

Various preparations containing acidifying agents are also widely used (such as "Ropal" from Germany, "Fadona" from Austria, "Magimix Light Green" from France, "Yasko Mill" from Turkey, etc.).

In addition to acidifying agents, preparations based on the bacteriocin nisin, synthesized by certain strains of lactococci subspecies Lactococcus lactis ssp. lactis, can be used. Nisin inhibits the growth of Gram-positive spore-forming bacteria, particularly at the spore germination stage. Such preparations include "Selectin" (Russia), "Nisaplin", and "Crisin" (imported products). Depending on the degree of flour contamination by spore-forming bacteria, the following dosages of Selectin have been established: prophylactic—0.10–0.15%; basic—0.25–0.30%; maximum—0.5–0.6% by mass of flour.

Biological methods for preventing ropy bread are more promising. Biological acidifying factors include liquid Yeast, liquid wheat sours, concentrated lactic acid sourdough (CLAS), and propionic acid bacteria.

Certain species of lactobacilli possess high antagonistic activity against the causative agents of ropy spoilage. The method of preparing a sourdough starter using lactic acid bacteria of the species Lactobacillus fermentum 27 has found wide industrial application.

Methods for destroying the spores of ropy bread pathogens involve the disinfection of equipment and premises of bakery production enterprises, as well as UV irradiation using BUV-15 or BUV-30 lamps installed at a distance of 20 cm from the object.

Molding is the most common type of microbiological spoilage in bread. Unlike flour microorganisms, mold Fungi represent a secondary infection that affects the finished product. Mold spores present in flour are generally destroyed during baking. Bread emerges from the oven practically sterile. However, during the cooling, transportation, and storage of bread, its surface becomes contaminated with mold spores. Molding most frequently occurs under improper bread storage conditions: tight packaging, elevated air temperatures (25–30 °C), and high relative humidity (70–85%). Nevertheless, mold development is also possible within a temperature range of 5 to 50 °C. From this perspective, only freezing bread products eliminates the possibility of molding. Product moisture or Water activity (a_w) is also significant.

Bread is most commonly spoiled by the growth of fungi belonging to the genera Aspergillus, Penicillium, Mucor, Rhizopus. The fungal mycelium spreads first across The surface of the bread and then penetrates into the crumb through cracks and pores. Molds form fluffy or velvety coatings of various colors On the surface of products: Aspergillus fumigatus—whitish-yellow; Aspergillus flavus—yellow-green; Aspergillus glaucum—gray-blue; Aspergillus ochraceus—yellow-orange; Aspergillus niger—black; Penicillium glaucum—bluish-green; Penicillium olivaceum—brown-yellow; Mucor mucedo—light gray; Mucor plumbeus—grayish-black; Rhizopus nigricans—white with black heads.

Bread affected by mold fungi acquires an unpleasant musty taste and odor due to the Breakdown of Proteins, fats, and CARBOHYDRATES by fungal Enzymes. Moldy bread is no longer fit for consumption. Furthermore, it may contain toxic substances known as mycotoxins.

Measures for Preventing Bread Molding. To prevent molding, bread must be stored in a dry, well-ventilated room at an air temperature not exceeding 10–12 °C and a relative humidity of 70–75%. Rapid cooling of bread immediately after baking prevents the molding process. Bread in trays and on racks should be stacked loosely, leaving gaps for air Circulation. The bread surface must be free of cracks and damage. Bread storage facilities should be equipped with supply and exhaust ventilation. Equipment and utensils used in the storage and transportation of finished products must be kept clean, thoroughly washed, and periodically disinfected.

During long-term bread storage, considerable attention is paid to The properties of packaging Materials. Cellophane, polyethylene, polypropylene films, and composite film materials based on polymer films, etc., are used for packaging bakery products. Packaging bread not only helps prevent molding but also preserves its freshness, makes it convenient for transportation and sale, and enhances its visual appeal to the consumer. Improving the barrier properties of packaging materials by combining them with antimicrobial additives is a promising approach. Polymer materials possessing microbicidal properties have already been developed through the incorporation of potassium sorbate and propionic acid salts into the polymer melt.

Chemical methods for inhibiting mold development involve The Use of organic acids (propionic, sorbic, citric, succinic, fumaric, etc.), as well as their potassium, calcium, and sodium salts. Chemical preservatives are generally introduced during dough mixing.

Chalk mold (chalk rot) of bread. The causative agents of chalk mold in bread are imperfect fungi of the species Endomycopsis fibuliger, Endomyces chodacii, and Hyphopichia burtonii (synonyms Trichosporon variabile, Candida variabile), Debaryomyces occidentalis, and Saccharomyces fibuligera. The development of these fungi on the bread crust surface and in the crumb results in a white, dry, powdery coating resembling chalk or flour dust.

Fungi of the genera Endomycopsis and Debaryomyces proliferate rapidly on organic substrates, including bread. Spores of these fungi are believed to be highly heat-resistant and survive the baking process.

Little attention has been paid in the literature to the "chalk mold" of bread due to its relatively rare occurrence. In recent years, however, cases of chalk mold have become more frequent with the intensive Introduction of sliced products into retail trade. While chalk mold is considered harmless to human health, bread affected by it loses its commercial appeal.